Adding a texture to follow the shape of the mesh
You can create multiple UV maps (click the add button alongside the list of UV maps in the mesh properties). You can then position your UV for the rust over the rush image without upsetting your ‘base’ UVs.

Your material will then need to be aware of both UV maps and mix between them as appropriate (eg, my using the Alpha of your rust image) although this depends on your choice of render engine (Blender Internal, Cycles).